Production of wetlaid-spunbond laminate membrane support

ABSTRACT

Nonwoven fabric laminates suitable for use as semipermeable membrane supports are produced by forming a spunbond nonwoven fabric first layer of continuous thermoplastic polymer filaments; forming a wet-laid nonwoven fabric second layer of discrete length thermoplastic polymer fibers; and bonding the first and second layers in opposing face-to-face relationship to form a composite support, where the first and second layers define first and second outer surfaces of the composite support. The resulting semipermeable membrane supports provide an advantageous balance of properties, including smoothness, porosity, interlaminar adhesion, and flux properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/615,231, filed Jul. 7, 2003, now U.S. Pat. No. 7,051,883 which ishereby incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention is directed to nonwoven fabrics suitable for useas filtration support media. The present invention is more specificallydirected to the production of nonwoven laminate fabrics suitable for useas precise filtration support media.

BACKGROUND OF THE INVENTION

Precise filtration processes, such as reverse osmosis, nano-filtration,ultra-filtration and micro-filtration, are used within a wide range ofapplications, including seawater desalination, fruit juice productionand industrial wastewater treatment, among others. Synthetic filtrationmembranes, commonly referred to as semipermeable membranes, aretypically used in conjunction with precise filtration processes.Semipermeable membranes provide selective mass transport, generallyallowing the molecules of the solvent, but not the solute, to passthrough their thickness. Semipermeable membranes are generally a thinlayer of polymer, such as a layer of cast film. Synthetic filtrationmembranes may be formed from a number of polymers, including celluloseacetate, polyamide, polysulfone, polyvinylidene fluoride polymers andthe like.

The process conditions encountered during filtration can be fairlyrigorous. For example, elevated pressures of up to about 1200 psi may beemployed to separate salt from seawater using reverse osmosis.Unfortunately, semipermeable membranes are typically relatively fragile.Laminate filter constructions incorporating a supporting substrate alongwith the semipermeable membrane are known to improve the durability offiltration media. Exemplary supporting substrates include a variety ofporous materials, including sintered materials and nonwoven fabrics.

Laminate filter constructions were traditionally formed by casting thesemipermeable membrane directly onto the supporting substrate. Morerecently, composite membranes, often referred to as Thin Film Composites(“TFC”) have been developed. Composite membranes include a porouspolymer layer, such as a porous polymer film, in addition to thesemipermeable membrane. The porous polymer layer is typically locatedbetween the semipermeable membrane and the supporting substrate. Theintermediate porous polymer layer allows much thinner semipermeablelayers to be used, yielding higher flux rates.

During filtration, the ingressing liquid stream is typically transportedthrough the semipermeable or composite membrane initially, exitingthrough the supporting substrate. Consequently, the supporting substratemust provide strength properties while having a minimal effect on thesemipermeable or composite membrane's transport properties, e.g.permeability or flux.

Suitable supporting substrates, e.g., suitable nonwoven fabrics, exhibita number of other advantageous properties, as well. For example, thesupporting substrate should exhibit acceptable adhesion to theintermediate porous polymer layer or semipermeable membrane, to avoiddelamination during filtration. Suitable adhesion may be achieved byallowing the intermediate porous polymer layer or semipermeable membraneto penetrate down into the surface of the supporting substrate. However,the penetration of the supporting substrate by the intermediate porouspolymer layer or semipermeable membrane represents a delicate balance.Inadequate penetration yields unacceptable adhesion within the filtermedia. Over penetration of the supporting substrate, e.g., penetrationby the intermediate porous polymer layer or semipermeable membrane tothe surface opposing the cast surface, results in uneven filtrationproperties (e.g. reduced flux) and/or damage of the semipermeablemembrane due to the partial excessive pressurization during filtration.

In addition to the properties described above, supporting substratesshould further advantageously provide a suitably smooth surface on whichto apply the intermediate porous polymer layer or semipermeablemembrane. Surface imperfections, particularly surface projections,create pinholes within the intermediate porous polymer layer and/or thesemipermeable membrane, detrimentally affecting filter performance.

Supporting substrates made from nonwoven wet-laid fibers have been foundto provide an advantageously smooth surface and acceptable affinity tosemipermeable membranes. Exemplary wet-laid nonwoven webs intended foruse as semi-permeable membrane supports are described in U.S. Pat. No.5,851,355 to Goettmann, hereby incorporated by reference. Supportingsubstrates formed from nonwoven wet-laid fiber webs have beencommercially available under the product name MEMBACK® nonwovens, fromBBA.

Composite support constructions may be used to improve the economics ofsemipermeable filtration media, especially filtration supportsincorporating wet-laid nonwoven webs. For example, U.S. Pat. Nos.4,728,394 and 4,795,559 describe membrane supports that include a cardedfiber layer bonded to a wet-laid web. However, although porous membranesincorporating carded webs provide a number of beneficial properties,such laminates can suffer from an unacceptable level of pinholes withinthe semipermeable or composite membrane. Carded webs are further arelatively expensive substrate.

Consequently, a need remains for composite supports incorporatingwet-laid fiber webs that provide improved surface properties. Therefurther remains a need for composite supports incorporating wet-laidwebs that can be produced more economically.

BRIEF SUMMARY OF THE INVENTION

The present invention provides composite supports for a semipermeablemembrane exhibiting improved filter performance due to theiradvantageous surface properties. The composite supports are produced bya method including the steps of forming a spunbond nonwoven fabric firstlayer of continuous thermoplastic polymer filaments; forming a wet-laidnonwoven fabric second layer of discrete length thermoplastic polymerfibers; and bonding the first and second layers in opposing face-to-facerelationship to form a composite support, wherein the first and secondlayers define first and second outer surfaces of the composite support.

The composite supports of the present invention include a wet-laid fiberweb along with a spunbond fabric. Surprisingly, composite supportsformed from wet laid and spunbond layers have been found to producefewer surface disparities within the resulting filtration media.

The composite supports of the invention generally include a first layerof spunbond nonwoven fabric formed of continuous thermoplastic polymerfilaments defining a first outer surface superposed with a second layerof wet-laid nonwoven fabric formed of discrete length thermoplasticpolymer fibers defining a second outer surface. In preferredembodiments, the composite supports further include a thermoplasticpolymer binder bonding the first and second layers to one another. Insuch embodiments, the thermoplastic polymer binder is in fibrous form.In further aspects, the thermoplastic polymer binder is adhered to thefilaments of the first layer and to the fibers of the second layer.

The continuous thermoplastic polymer filaments and discrete lengththermoplastic polymer fibers may each independently be formed from anumber of resins, including polyester and polyamide, and copolymers andmixtures thereof. In preferred embodiments of the invention, thecontinuous filaments of the first layer and the discrete length fibersof the second layer are formed of the same thermoplastic polymer.

For example, the continuous thermoplastic polymer filaments and discretelength thermoplastic polymer fibers may both be formed from polyesterpolymer. In further aspects, the thermoplastic polymer binder comprisesa polyester copolymer having a lower melting temperature than thepolyester polymer used to form the filaments and discrete length fibers.The thermoplastic polymer binder may further be formed from a mixture ofhigher melting and lower melting polyester copolymers.

The discrete length fibers within the wet-laid nonwoven fabric typicallyhave a length of from about 2.5 to 40 mm and are from about 0.2 to 3.0denier per filament (dpf). The filaments of the spunbond layer generallyare from about 1 to 10 denier per filament. The spunbond nonwovengenerally has a basis weight of about 10 to 35 gsm and the wet-laidnonwoven fabric typically has a basis weight of about 30 to 70 gsm, withthe resulting composite support having an overall basis weight of up to80 gsm.

Filtration devices may be formed in accordance with the invention byadhering either a composite membrane or a semipermeable membrane to thesecond outer surface the composite support, i.e. the outer surface ofthe wet-laid layer. Exemplary materials from which to form thesemipermeable membrane include cellulose acetate (“CA”), cellulosetriacetate, CA-cellulose triacetate blends, gelatin, polyamine,polyimide, poly(ether imides), aromatic polyamide, polybenzimidazole,polybenzimidazolone, polyacrylonitrile (“PAN”), PAN-poly(vinyl chloride)copolymer, polysulfone, polyethersulfone, poly(dimethylphenylene oxide),poly(vinylidene fluoride), polyelectrolyte complexes, polyolefins,poly(methyl methacrylate) and copolymers thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention will be made apparent from the following detailed descriptionof the invention and from the drawings, in which:

FIG. 1 is a schematic illustration of an enlarged cross-sectional viewof exemplary filtration media formed in accordance with the invention;

FIG. 2 is a schematic illustration of an exemplary process for formingfiltration media in accordance with the present invention;

FIG. 3 is a schematic illustration of an apparatus for forming themembrane support of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

A cross-sectional view of exemplary filtration media in accordance withthe present invention is provided in FIG. 1. The filtration media 10generally includes a composite membrane 11 adhered to a compositesupport 14. The composite membrane 11 generally includes a semipermeablemembrane 12 and intermediate porous polymer layer 13. In alternativeembodiments of the invention, the semipermeable membrane 12 may be usedwithout the intermediate porous polymer layer 13, i.e. the intermediateporous polymer layer may be omitted from the filtration media 10. Thecomposite support 14 includes (a) a first layer 16 formed of spunbondfabric defining a first outer surface 17 and (b) a second layer 18formed of wet-laid fabric defining a second outer surface 22, i.e., thecasting surface.

Although the filtration media, composite membranes and compositesupports of the invention are referred to as containing “layers,” thisterm is merely used to facilitate discussion concerning the differingcompositions and/or constructions which may be present in variousregions within the filtration media, composite membrane or compositesupport thickness. The filtration media, composite membranes andcomposite supports of the present invention, although referred to asbeing formed from such “layers,” nevertheless provide unitary structuresexhibiting cohesive properties throughout their thickness. Further, each“layer” is typically in direct liquid communication with its adjacentlayer(s).

The spunbond fabric which forms the first layer 16 includes a pluralityof continuous thermoplastic polymer filaments. More particularly, thespunbond fabric typically includes from about 80 to 100% weight percentcontinuous thermoplastic polymer filaments. As used herein, the terms“filament” and “continuous filament” are used in a generic sense torefer to fibrous materials of indefinite or extreme length, such as alength of several feet or greater. The denier per filament (“dpf”) ofthe continuous filaments within the first layer 16 typically ranges fromabout 1 to 10 dpf, such as from about 4 to 6 dpf. In certain preferredembodiments, the spunbond filaments within the first layer 16 have afineness of about 4 dpf, particularly 4 dpf fibers with a trilobal crosssectional shape. In alternative embodiments, the spunbond filaments mayhave a mixture of deniers.

The continuous filaments within the spunbond first layer 16 may beformed from any fiber forming thermoplastic polymer providing acceptablemechanical properties and chemical resistance. For example, thecontinuous filaments may be formed from polyester homopolymers and/orcopolymers, or from polyamide homopolymers and/or copolymers or mixturesthereof. An exemplary polyester is polyethylene terephthalate. Exemplarypolyamides include nylon 6 and nylon 6,6. In preferred embodiments ofthe invention, the continuous filaments within the first layer 16 areformed from polyethylene terephthalate.

The first layer 16 may be formed from spunbond continuous filaments ofvarious cross sections known in the art of fiber formation, includingtrilobal, quadlobal, pentalobal, circular, elliptical anddumbbell-shaped. Either a single cross-section or a mixture of filamentsof differing cross section may be included within the first layer 16. Inpreferred embodiments of the invention, the first layer 16 is formedfrom spunbond filaments having a trilobal cross section.

Applicants have found that spunbond layers possessing fairly uniformstructures provide an unexpectedly smooth casting surface 22.Surprisingly, the smoothness of the interfacial surface 20 of thespunbond layer 16 translates into a superior casting surface 22 on theopposing side of the wet-laid second layer 18.

The first layer 16 further provides beneficial transport properties,e.g. porosity-permeability values. Exemplary porosity-permeabilityvalues for the first layer 16 ranges from about 800 to 1550 beforecalendering, such as porosity-permeability values ranging from about1400 to 1550 (Textest Air Permeability). Porosity-permeability isgenerally related to density, such as apparent density, with higherdensity materials typically exhibiting lower porosity-permeabilityvalues. Exemplary apparent densities for the first layer 16 prior tocalendering generally range from about 0.100 g/cc to 0.250 g/cc, such asapparent densities ranging from about 0.100 g/cc to 0.150 g/cc. Theadvantageous porosity-permeability values of the invention areunexpected in light of the density of the first layer 16. Although notwishing to be bound by theory, Applicants believe that such beneficialporosity-permeability values may be due the more open fiber structuresprovided by the continuous thermoplastic filaments in comparison toother nonwovens of comparable density.

To provide adequate interlaminar strength within the first layer 16, thecontinuous filaments within the spunbond first layer 16 are bonded toeach other at points of contact. Although the continuous filamentswithin the spunbond first layer 16 are bonded, the nonwoven structureremains sufficiently open to provide beneficial flux properties, asindicated by the throughputs described above. However, although bondedat a level to insure adequate flux, the first layer 16 is neverthelessconsidered to be substantially fully bonded in that the spunbondfilaments are bonded together at a plurality of crossover points. Thebonding within the first layer 16 can be accomplished by any knownmeans, such as by the melting of thermoplastic binder filaments,thermoplastic resin bonding, etc. In preferred embodiments, the firstlayer 16 is bonded using binder filaments. The binder filaments may beincluded within the first layer 16 in any amount effective to induce anadequate level of bonding. The binder filaments are typically present inthe first layer 16 in an amount ranging from about 2 to 20 weightpercent, such as an amount of about 10 weight percent. In alternativeaspects of the invention, the spunbond filaments within the first layer16 may be multiconstituent fibers that include a thermoplastic binderpolymer as a component. For example, in such alternative embodiments thespunbond filaments may have a sheath/core configuration in which thesheath is formed from a binder polymer.

The binder filaments within the first layer 16 are generally formed fromany polymer exhibiting a melting or softening temperature at least about10° C. lower than the continuous filaments. The binder filaments may allbe formed from the same polymer or may include a mixture of higher andlower melting binder filaments. For example, the binder filaments mayinclude a mixture of filaments, a first portion of which have a lowermelting temperature, such as about 225° F., and a second portion ofwhich have a higher melting temperature, such as about 375° F. Exemplarybinder filaments may be formed from one or more lower melting polymersor copolymers, such as polyester copolymers. In one embodiment of theinvention, the spunbond layer is produced by extruding polyesterhomopolymer matrix filaments (polyethylene terephthalate) interspersedwith binder filaments formed from a lower melting polyester copolymer,particularly polyethylene isophthalate.

The binder filaments within the first layer 16 may have anycross-section known in the art. In preferred embodiments, the binderfilaments within the first layer 16 have a trilobal cross-section. Thebinder filaments within the first layer 16 may further have any denieror mixture of deniers known in the art for binding spunbond fabrics.

The first layer 16 is typically characterized by a basis weight rangingfrom about 10 to 35 gsm, such as 12 to 25 gsm. Suitable spunbond fabricsfor use as the first layer 16 are commercially available, such as REEMAYStyle 2004 spunbond, from Reemay, Inc. of Old Hickory, Tenn.

In the embodiment illustrated in FIG. 1, the composite support 14includes a single spunbond first layer 16. In alternative embodiments,the composite support 10 can include more than a single spunbond layer.For example, the composite support 10 may include two contiguousspunbond layers. For embodiments including at least two spunbond layers,the fibers and materials comprising the respective spunbond layers maybe the same or may differ. For example, the spunbond layers may differin composition, denier, basis weight or fiber cross-section.

The second layer 18 is a wet-laid nonwoven fabric formed from aplurality of discrete length thermoplastic polymer fibers. Moreparticularly, the wet-laid nonwoven fabric typically includes discretelength thermoplastic polymer fibers in amounts ranging from about 80 to100 weight percent. As used herein the term “discrete length fibers” isused in a generic sense to describe fibrous materials which are notcontinuous in nature. Exemplary non-continuous fibers include staplefibers, wet-laid fibers and melt-blown fibers. Exemplary average lengthsfor the discrete length thermoplastic polymer fibers within the secondlayer 18 generally range from about 2.5 to 40 mm, preferably from about5 to 13 mm. In preferred embodiments, substantially all of the discretelength fibers have approximately the same length, such as a lengthranging from about 5 mm to 13 mm. In alternative aspects of theinvention, a mixture of discrete fiber lengths may be employed.

One important feature of the wet-laid second layer 18 is itspermeability-porosity. More particularly, if the permeability-porosityof the second layer 18 is too low, the semi-permeable membrane 12 willnot bond to the composite support 14. Consequently, thepermeability-porosity of the second layer 18 typically ranges from about5 to 30 cfm (Textest Air Permeability after calendering). Along withfiber length (described above), the permeability-porosity within thesecond layer 18 is influenced by the fiber stiffness, which is in turnreflected by the fiber denier. Accordingly, the denier per filament(“dpf”) of the discrete length fibers within the second layer 18typically ranges from about 0.2 to 3.0 dpf, such as from about 0.43 to1.5 dpf. In certain embodiments, the discrete length fibers all haveapproximately the same denier. In alternative embodiments, discretelength fibers having a mixture of deniers may be employed within thesecond layer 18.

The discrete length fibers within the second layer 18 may be formed fromany fiber forming thermoplastic polymer providing acceptable mechanicalproperties and chemical resistance. For example, the discrete lengthfibers may be formed from homopolymers of polyester or polyamide, ormixtures or copolymers thereof. An exemplary polyester from which thediscrete length fiber may be formed is polyethylene terephthalate.Exemplary polyamides include nylon 6 and nylon 6,6. In preferred aspectsof the invention, the discrete length fiber is formed from polyethyleneterephthalate.

The discrete length fibers within the second layer 18 may have anycross-section known in the art of fiber formation. In preferredembodiments, the discrete length fibers have a circular cross-section.In alternative embodiments, the discrete length fibers may have across-section imparting greater stiffness. Exemplary stiff fiber crosssections include any non-circular fibers defining four or more lobes,i.e., quadralobal (cross-shaped), pentalobal and the like, having anysuitable modification ratio or dimensional relationship. The secondlayer 18 may be formed from discrete length fibers having a singlecross-sectional configuration. Alternatively, the discrete length fiberswithin the second layer 18 may include a mixture of cross-sectionalconfigurations.

The discrete length fibers are bonded to each other at points ofcontact, but the second layer 18 remains sufficiently open to providebeneficial transport properties. The second layer 18 is considered to besubstantially fully bonded in that the discrete length fibers are bondedtogether at a plurality of crossover points. The bonding within thesecond layer 18 can be independently accomplished by any known means,such as by the melting of binder fibers, resin bonding, etc. Inpreferred embodiments, the second layer 18 is bonded using binder fibersand thus further includes binder fibers. The binder fiber may beincluded within the second layer 18 in any amount effective to induce anadequate level of bonding. The binder fiber is typically present in thesecond layer 18 in amounts ranging up to about 60 weight percent, suchas in amounts ranging up to about 40 weight percent. In alternativeaspects of the invention, the discrete length fibers within the secondlayer 18 are multiconstituent fibers that include a binder polymer as acomponent. For example, in such alternative embodiments the discretelength fibers may have a sheath/core configuration in which the sheathis formed from a binder polymer.

The binder fibers within the second layer 18 are generally formed fromany polymer exhibiting a melting or softening temperature at least about10° C. lower than the discrete length fibers. The binder fibers may allbe formed from the same polymer or may include a mixture of higher andlower melting binder fibers. For example, the binder fibers may includea mixture of fibers, a first portion of which have a lower meltingtemperature, such as about 225° F., and a second portion of which have ahigher melting temperature, such as about 375° F. Exemplary binderfibers may be formed from one or more low melting polyolefin polymers orcopolymers, one or more low melting polyester polymers or copolymers ormixtures thereof. In preferred embodiments of the invention, the binderfiber is formed from a low melting polyester copolymer, particularlypolyethylene isophthalate.

The binder fibers within the second layer 18 may have any cross-sectionknown in the art. In preferred embodiments, the binder fibers within thesecond layer 18 have a circular cross-section. The binder fibers withinthe second layer 18 may further have any denier or mixture of deniersknown in the art for binding nonwoven fabrics.

The materials and process conditions associated with the second layer 18are selected so as to provide a smooth casting surface 22. The secondlayer 18 typically provides a porosity-permeability after calenderingranging from about 5 to 30 cfm Textest Air Permeability, depending onthe substrate's performance need. The second layer 18 is typicallycharacterized by a basis weight ranging from about 30 to 70 gsm, such asfrom about 40 to 60 gsm. Suitable wet-laid fabrics for use as the secondlayer 18 have been commercially available, such as MEMBACK® nonwovens,from BBA.

In the embodiment illustrated in FIG. 1, the composite support 14includes a single wet-laid second layer 18. In alternative embodiments,the composite support 14 includes more than a single wet-laid layer. Forexample, the composite support 14 may include two contiguous wet-laidlayers. For embodiments including at least two wet-laid layers, thefibers and materials comprising the respective wet-laid layers may bethe same or may differ. For example, the wet-laid layers may differ incomposition, average denier, basis weight or fiber cross-section.

The composite support 14 formed by the combination of the first layer 16and the second layer 18 generally has a thickness ranging fromapproximately 2 to 8 mils, such as a thickness of about 3 to 4 mils. Thecomposite support 14 is further typically characterized by a basisweight of less than about 80 gsm, such as a basis weight ranging fromabout 40 to 70 gsm. The composite support 14 generally provides aporosity-permeability ranging from about 5 to 30 cfm Textest AirPermeability.

As shown in FIG. 1, the filtration media 10 of the invention furtherincludes a composite membrane 11, adhered to the casting surface 22 ofthe composite support 14. The composite membrane 11 includes asemipermeable membrane 12 and an intermediate porous polymer layer 13.

Any intermediate porous polymer layer 13 known in the art of precisefiltration may be used in conjunction with the composite support 14.Polysulphone is one example of material which may be used to form theintermediate porous polymer layer 13. As known in the art, intermediateporous polymer layers typically have a cellular structure that resemblestiny tubes extending from one plane to the next. The intermediate porouspolymer layer 13 generally improves the surface smoothness, allowing theuse of thinner semipermeable membranes 12, thereby increasingthroughput. The intermediate porous polymer layer 13 typically ranges inthickness from about 40 to 70 microns, such as from about 45 to 65microns, particularly from about 45 to 50 microns.

Any semipermeable membranes known in the art of reverse osmosis,ultrafiltration, nanofiltration or micro-filtration may be used inconjunction with the composite support 14. Non-limiting exemplarysemipermeable membranes include polymeric films formed from celluloseacetate (“CA”), cellulose triacetate, CA-cellulose triacetate blends,gelatin, polyamine, polyimide, poly(ether imides), aromatic polyamide,polybenzimidazole, polybenzimidazolone, polyacrylonitrile,PAN-poly(vinyl chloride) copolymer, polysulfone, polyethersulfone,poly(dimethylphenylene oxide), poly(vinylidene fluoride),polyelectrolyte complexes, polyolefins, poly(methyl methacrylate) andcopolymers and mixtures of these materials.

Semipermeable membranes suitable for use with the present invention mayhave any thickness known in the art for such membranes, such as athickness ranging from about 25 angstroms to 100 microns, preferablyabout 1 micron. In some embodiments, the semipermeable membranes areasymmetrical in nature.

In the embodiment illustrated in FIG. 1, the filtration media 10includes a single semipermeable membrane 12 and a single intermediateporous polymer layer 13. In alternative embodiments of the invention,the filtration media may include multiple semipermeable membrane layersand/or multiple intermediate porous polymer layers. In such embodiments,each of the semipermeable membrane and/or porous polymer layers may bethe same or may differ is some respect, such as differing compositionsor configurations.

The composite membrane 11 impregnates at least the outermost surface ofthe composite support 14 to provide adequate adhesion to the resultingfiltration media 10. However, although impregnating the outermost regionof the composite support 14, the composite membrane 11 does notover-penetrate the composite support 14. For example, the compositemembrane 11 does not penetrate through the entire thickness of thecomposite support 14, i.e., to the outer surface 17 of the first layer16. The absence of such over-penetration is surprising in light of themore open fiber structure provided by the continuous filaments withinthe spunbond layer 16 in comparison to nonwoven webs formed ofdiscontinuous filaments, such as staple fibers.

Applicants hypothesize that the continuous filaments within the spunbondfirst layer 16 result in a smoother casting surface 22 in comparison toconventional composite supports incorporating carded nonwovens. Althoughnot wishing to the bound by theory, Applicants have found that thesurface roughness of intermediate surfaces within the composite support,such as the surface of the spunbond layer contacting the wet-laid layer18, ultimately affects the surface properties of the opposing surface ofthe wetlaid layer, i.e. the casting surface 22. The superior smoothnessimparted by the continuous filaments within the first layer 16 isfurther surprising in light of the fact that carded staple fiber webs,i.e., webs formed of longer discrete length fibers, impart greatercasting surface roughness to composite supports in comparison tomembrane supports formed of a single layer of shorter discrete lengthfibers, i.e., wet-laid fibers. The smoother casting surfaces of theinvention generally result in fewer holes and/or voids within thecomposite membrane 11.

The absence of holes and voids within the composite membrane 11 isgenerally reflected by higher efficiencies within the filtration media.Higher efficiencies are typically evidenced by a combination of elevatedfiltrate rejection characteristics and permeate throughputs.

The filtration media 10 may be formed using manufacturing processesknown in the industry. Referring now to FIG. 2, an illustrative processfor forming advantageous embodiments of the filtration media 10 isprovided. As shown, the composite support may be produced by (1) formingthe first layer via a spunbond process and the second layer via awet-laid process, 24 and 24 a, respectively; (2) bonding the spunbondand wet-laid fabrics to form a composite support, 26; and (3) applying acomposite membrane to the composite support, 28.

The first layer can be produced using any conventional spunbondingapparatus capable of forming a nonwoven fabric from substantiallycontinuous thermoplastic polymer filaments and binder filaments.Spunbonding generally involves extruding and subsequently attenuatingcontinuous filaments as they are being deposited onto a movingcollection surface or screen. The filaments collect in the form of aweb, which is then conveyed on the screen to a thermal fusion station,preferably a pair of cooperating calender rolls, to provide a spunbondfabric. The web is bonded together to provide a multiplicity of thermalbonds distributed throughout the spunbond fabric. The bonded spunbondfabric is then wound up by conventional means on a roll. Spunbondingprocesses and apparatus are well known to the skilled artisan.

As indicated in FIG. 2, the second layer is typically formed in aseparate wet-laying process. Any wet-laid process known in the art maybe used to form the second layer. Wet-laying processes generally involvedepositing a layer of fibers suspended within an aqueous slurry,commonly referred to as a furnish, onto a continuous screen. Inpreferred embodiments, the fibers within the wet laid layer are randomlydeposited to give the web isotropic properties that are nondirectionalin nature. Water from the furnish is drawn through the screen, leavingbehind an initial wet-laid web. A stack of drying rollers removesadditional water from the initial wet-laid web and consolidates the web.The dried wet-laid web exits the drying rollers and is wound up byconventional means on a roll. Wet-laid processes and apparatus are knownto the skilled artisan and are disclosed, for example in U.S. Pat. No.5,851,355 to Goettmann.

As shown in FIG. 2, the composite support 14 is subsequently produced bybonding the preformed spunbond and wet-laid layers. Advantageously, thelayers are bonded together to provide a multiplicity of thermal bondsbetween the spunbond and wet-laid fabrics. A plurality of verticallystacked rolls may be used to bond the preformed spunbond and wet-laidlayers, as illustrated in FIG. 3. As shown, the vertically stacked rollsdefine multiple nips, and the preformed layers pass through the multiplenips in a serpentine pattern. Each nip within the bonding apparatus ofFIG. 3 may be independently heated and loaded. In alternativeembodiments, a series of horizontal rolls may be used to form multiplebonding nips, each of which may similarly be independently heated andloaded.

As shown in FIG. 3, the spunbond 80 and wet-laid 82 webs are unwoundfrom rolls 84 and 86, respectively. The rolls of spunbond web 84 andwet-laid web 86 are arranged so that, upon unwinding, the spunbond 80and wet-laid 82 layers are superposed in an opposing face-to-facerelationship.

The superposed layers 88 are subsequently conveyed longitudinallythrough a first nip 90. Within the first nip 90, the binder filaments inthe spunbond fabric and the binder fibers in the wet-laid web begin tosoften and fuse to adhere the layers together. The first nip 90 isconstructed in a conventional manner as known to the skilled artisan. Inthe embodiment illustrated in FIG. 3, the first nip 90 is defined by apair of cooperating calender rolls 94 and 96, which are preferablysmooth and advantageously formed from steel. The cooperating calenderrolls 94 and 96 preferably provide a fixed gap nip. The fixed gap nipensures that the superposed layers 88 will not exit the first nip 90thinner than the targeted gap thickness, regardless of any excesspressure that may be applied. In the advantageous embodiment illustratedin FIG. 3, pressure is applied to the first nip 90 using a topmost roll97.

Bonding conditions, including the temperature and pressure of the firstnip 90, are known in the art for differing polymers. For compositesupports comprising polyethylene terephthalate nonwoven spunbond andwet-laid fabrics which both further include polyethylene isophthalatebinder filaments and/or fibers, the first nip 90 is preferably heated toa temperature between about 120° C. and 230° C., preferably from about200 to 225° C. The first nip 90 is typically run at pressures rangingfrom about 40 to 350 pounds per linear inch (pli), such as from about 80to 200 pli.

In an alternative embodiment, shown by broken lines, the two superposedlayers 88 can be partially wrapped around an additional roll, e.g.passing over the top roll 97 and then through the nip defined betweenrolls 97 and 94, which is heated to a temperature of about 200° C. priorto passing through the nip 90 between rolls 94, 96. Passing thesuperposed webs 88 over the additional heated roll 97 prior to thecalender rolls 94, 96 preheats the superposed layers 88 before theyenter the nip 90. Such preheating allows increased bonding speeds.

Returning now to FIG. 3, the superposed layers exiting the first nip 90subsequently enters a second nip 98. The second nip 98 is formed by atop roll 96 and a bottom roll 104. The rolls 96 and 104 are preferablysteel.

The pressure within the second nip 98 is typically higher than thepressure in the first nip 90, further compressing the superposed layersexiting the first nip 90. Consequently, the gap formed by the second nip98 is narrower than the gap provided by the first nip 90. The pressurein the second nip 98 is typically about 120 to 1100 pli, such as fromabout 180 to 320 pli. The second nip 98 may further be heated, such asto a temperature ranging from about 120 to 230° C., preferably fromabout 200° C. to 225° C. The resultant bonded composite support 14exiting the second nip 98 may be transported over a chill roll 106 andwound up by conventional means on a roll 112.

Although a bonding apparatus in the form of a series of calender rollsis illustrated in FIG. 3, other bonding apparatus such as ultrasonic,microwave or other RF treatment zones which are capable of bonding thesuperposed layers can be substituted for the calender rolls of FIG. 3.Such conventional thermal treatment stations are known to those skilledin the art and are capable of effecting substantial thermal fusion ofthe two nonwoven webs. It is also possible to achieve bonding throughthe use of an appropriate bonding agent as is known in the art, singlyor in combination with thermal fusion.

Returning now to FIG. 2, the filtration media is then formed by castingor otherwise applying or coating the composite or semipermeable membraneonto the composite support. Methods by which to apply composite andsemipermeable membranes to porous supports are known to the skilledartisan and are disclosed, for example in U.S. Pat. No. 4,277,344 toCadotte; U.S. Pat. No. 5,522,991 to Tuccelli et al. and U.S. Pat. No.6,132,804 to Rice et al. In general, the composite or semipermeablemembrane may be applied to the composite support by means such as dipcoating, extrusion coating, knife-over-roll coating, slot coating andthe like. The thickness of the composite or semipermeable membrane mayvary widely, depending upon the specific membrane composition andfiltration application, as known in the art. Subsequent to coatingapplication, the composite or semipermeable membrane is subjected to asolification process to bond the composite or semipermeable membrane tothe composite support. Various solidification processes are known to theskilled artisan and may be employed in conjunction with the presentinvention. Exemplary solidification processes include hot air drying,interfacial polymerization, crosslinking and the like. The resultingfiltration media exits the solidification process and is wound up byconventional means on a roll.

In embodiments of the invention directed to composite membranes ormultiple semipermeable membrane layers, the various layers may beapplied to the composite support using consecutive coating processes,such as consecutive slot coatings, as described in U.S. Pat. No.6,132,804 to Rice et al. In such embodiments, further preservation stepsmay be required to assure maintenance of the pore structure provided bythe interior membrane layers, as known in the art.

The particular membrane support employed will typically be determinedeither by the type of separation/filtration process in which it is usedand/or the requirements of the semipermeable or composite membranecasting process. Regardless of substrate configuration, porosity is animportant property for a properly functioning membrane support.Substrate thickness is another important factor to consider, becausethickness affects the total membrane area that can be accommodated intoa filtration module. Generally, thinner membrane supports allow greatermembrane area within a filtration module, equating to a higher moduleoutput. The use of lighter weight membrane supports further yieldssignificant cost savings to the user. In addition to suitable porosityand minimal thickness, composite supports of the inventionadvantageously provide uniformity in their thickness, have good adhesionto the composite or semipermeable membrane, have a minimal number ofsurface defects which could lead to pinholes and are strong enough towithstand the membrane casting process.

The composite supports of the invention may be advantageously used toform filtration media, particularly semipermeable membrane filtrationmedia employed in reverse osmosis, ultrafiltration and nanofiltrationapplications. However, the nonwoven laminates of the invention may alsobe suitable for a number of non-filtration applications, as well. Forexample, the nonwoven laminates of the invention may be employed in anyapplication in which a strong, smooth material is desired. Particularlyadvantageous non-filtration applications for the nonwoven laminates ofthe invention include banner and signage stock.

The following examples are provided for purposes of further illustratingspecific embodiments of the invention. It should be understood, however,that the invention is not limited to the specific details given in theexamples.

EXAMPLES

Examples 1 through 5 in accordance with the present invention wereproduced using the layer compositions provided in Tables 1 and 2 below.The samples below were produced from polyester spunbond and wetlaidfabrics that further included polyester binder fiber.

The individual wetlaid and spunbond fabric layers were prepared usingprocesses well known in the art. The wetlaid and spunbond fabric layerswere bonded into a composite support using the process described inconjunction with FIG. 3. The pressure between the thermal bonding rollsranged from about 80 to 200 pli, while the temperature of the thermalbonding nip was about 225° C. The pressure between the surfacecompaction rolls ranged from about 180 to 320 psi, while the temperatureof the surface compaction nip was about 223° C.

TABLE 1 Wetlaid Fabric Construction Layer Identification C D Basis Wt(gsm) 37.0 55.0 Fiber 1¹ Denier 0.4 0.4 Length (mm) 10.0 10.0 WeightPercent (%) 20.0 35.0 Fiber 2² Denier 1.5 1.5 Length (mm) 12.5 12.5Weight Percent (%) 38.0 25.0 Fiber 3¹ Denier 1.0 1.0 Length (mm) 5.0 5.0Weight Percent (%) 37.0 35.0 Fiber 4¹ Denier 2.0 2.0 Length (mm) 5.0 5.0Weight Percent (%) 5.0 5.0 ¹Commercially available from Kuraray Co.,Ltd. of Osaka Japan. ²Commercially available from Kosa of Charlotte,North Carolina.

TABLE 2 Spunbond Fabric Construction Layer Identification E³ F³ G³ BasisWeight (gsm) 13.6 18.0 34.0 Denier 4 2.2 2.2 ³Commercially availablefrom Reemay, Inc. of Old Hickory, Tennessee.

Table 3 provides the layer configuration and properties exhibited byExamples 1 through 5 and Comparative Examples 1 and 2. The basis weight,thickness, air permeability, bubble point and mean flow pore size wereall determined using methods well known in the art.

TABLE 3 Sample Performance Comp. Comp. Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex. 3 Ex.4 Ex. 5 Wetlaid layer A¹ B¹ C C D D D Spunbond layer None None E F E G FSample Basis 84 77 51.8 55.9 71.3 74.1 77.5 Weight (gsm) Thickness 4 3.93.3 2.7 3.6 3.2 3.5 (mils) Air Perme- 9 10.5 29.4 19.8 7.1 12.9 6.9ability (cfm) Bubble Point 23.5 45.3 46.8 35.4 28.8 33.0 25.5 (um) MeanFlow Pore 11 16.3 23.2 25.6 14.1 20.0 11.3 Size (um) ¹A and B arecommercially available wetlaid substrates.

As indicated in Table 3, membrane supports formed in accordance with theinvention are generally thinner and lighter than membrane supportsformed from wetlaid nonwoven alone. The membrane supports of theinvention further provide acceptable porosity, as indicated by the airpermeability, bubble point and mean flow pore sizes shown in Table 3.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method of producing a composite support for a semipermeablemembrane, comprising forming a spunbond nonwoven fabric first layerformed of continuous thermoplastic polymer matrix filaments and binderfilaments of a lower-melting thermoplastic polymer composition; forminga wet-laid nonwoven fabric second layer of discrete length thermoplasticpolymer fibers, including matrix fibers and binder fibers of alower-melting thermoplastic polymer composition; and bonding said firstand second layers in opposing face-to-face relationship to form acomposite support, whereby the first and second layers define first andsecond outer surfaces of the composite support.
 2. A method of producinga composite support according to claim 1, wherein said bonding furthercomprises transporting the composite support through a nip between apair of cooperating calender rolls.
 3. A method of producing a compositesupport according to claim 1, wherein the calender rolls are at atemperature between about 120° C. and 230° C. and the nip exertspressures ranging from about 80 to 200 pli.
 4. A method of producing acomposite support according to claim 2, wherein said bonding furthercomprises transporting the composite support through a second nipbetween a second pair of cooperating calender rolls.
 5. A method ofproducing a composite support according to claim 4, where the second nipis at a temperature between about 180° C. to 320° C. and the second nipexerts pressures ranging from about 150 to 260 psi.
 6. A method ofproviding a composite support according to claim 4, wherein said secondnip exerts a higher pressure than said first nip.
 7. A method ofproviding a composite support according to claim 4, wherein the gapwithin the second nip is narrower than the gap within the first nip. 8.A method of producing a composite support for a semipermeable membrane,comprising forming a nonwoven fabric first layer by extruding amultiplicity of continuous thermoplastic polymer matrix filaments andbinder filaments of a lower-melting thermoplastic polymer com position,randomly depositing the matrix and binder filaments on a collectionsurface, and bonding the matrix and binder filaments together to form aspunbond nonwoven web; forming a nonwoven fabric second layer bywet-laying discrete length thermoplastic polymer fibers, includingmatrix fibers and binder fibers of a lower-melting thermoplastic polymercomposition, to form a web and bonding the matrix and binder fiberstogether to form a wet-laid nonwoven web; arranging the first and secondlayers in an opposing face-to-face relationship and directing the layersthrough a series of heated nips to bond the first and second layers toone another.
 9. A method of producing filtration media, comprisingforming a spunbond nonwoven fabric first layer of continuousthermoplastic polymer matrix filaments and binder filaments of alower-melting thermoplastic polymer composition; forming a wet-laidnonwoven fabric second layer of discrete length thermoplastic polymerfibers, including matrix fibers and binder fibers of a lower-meltingthermo plastic polymer composition; bonding said first and second layersin opposing face-to-face relationship to form a composite support,whereby the first and second layers define respective first and secondouter surfaces of the composite support; and applying a semipermeablemembrane or porous polymer layer to the second outer surface of thecomposite support.
 10. A method of producing filtration media accordingto claim 9, wherein said applying step comprises at least one processselected from the group consisting of dip coating, extrusion coating,knife-over-roll coating and slot coating.